Domestic and industrial wastewater can be relatively rich in phosphorous, nitrogen, and carbonaceous contaminants that are essential nutrients for the growth of organisms. Phosphorus bans or limitations in synthetic detergents or changes in detergent formulas by the manufacturers have served to reduce the levels of phosphorous in domestic wastewater. However, many industrial and food process waste streams are inherently high in phosphorous content. One such example of high phosphorous content is the wastewater produced from a potato processing facility. The discharge of a phosphorous-rich wastewater into a receiving stream may stimulate accelerated algal growth, which can result in oxygen depletion and stream eutrophication.
Phosphorous is most efficiently removed from wastewater when it is in the form of orthophosphate (H2PO4−, HPO42−, or PO43−), as many orthophosphate salts are not soluble in water. Although organic phosphonates and condensed phosphates can be readily converted into orthophosphate by treating them with strong, hot oxidizing acid conditions, this is not practical on a million gallon per day scale of wastewater treatment. Fortunately, many microbiological processes perform the conversion of the organic and condensed phosphates to orthophosphate.
Traditional methods to remove solids, such as settling, filtration, and centrifuging, will remove the great majority of all insoluble phosphorous species with the solid fraction. Removal of the remaining soluble phosphorous species, which is the focus of this invention, has been performed using the following methods: removal by phosphate-accumulating microorganisms; precipitation by a metal ion; and precipitation as struvite.
Soluble phosphorous can be removed by aerobic or facultative microorganisms that can incorporate the phosphorus into their cell mass. Once incorporated within the biomass, the phosphorous can be removed from the system as waste activated sludge.
Other common phosphorous removal methods involve the formation of insoluble phosphate salts with calcium, aluminum, or iron and allow the resulting particles to settle with the clarified sludge. Vanotti et al. (U.S. Pat. No. 6,893,567) describes a process for phosphorous removal that involves the use of lime (CaO, calcium oxide) under conditions of low nitrogen and carbonate alkalinity. Dissolution of lime in water produces calcium ions and a solution pH around 11. The calcium ions bond to the orthophosphate ions to form insoluble calcium hydroxyphosphate (hydroxyapatite) that settles to the bottom of the clarifier.
Aluminum hydroxyphosphate is formed from alum treatment of the orthophosphate containing wastewater. Alum (aluminum sulfate) reacts with water and orthophosphate to form the insoluble aluminum hydroxyphosphate. The pH for successful phosphate removal by alum treatment lies in a very narrow range around 6 and two molecules of alum are required for every one of phosphate removed. At pH below 5 or above 7, the removal of phosphate is either incomplete or a fine solid forms which has poor settling qualities.
Iron, in the form of ferrous or ferric salts, has been used to precipitate orthophosphate as the insoluble iron hydroxyphosphate. Soluble iron ions combine with alkalinity and phosphate to form the insoluble ferric hydroxyphosphate and the floc ferric hydroxide, which helps in the rate of settling. The ideal wastewater pH for this to occur is around 6. Most of the sludge from this removal is moved into an anaerobic digester. A very common iron salt for this purpose is ferric chloride.
The use of alum or ferric chloride for phosphorous removal has a significant drawback relating to the acidity of the coagulants that are being added to the wastewater. Since both aluminum sulfate and ferric chloride are acidic in nature, they will impart a decreased pH to both the settled solids and the overflow water. By decreasing the pH, microbial activity in the digester (aerobic or anaerobic) or in the secondary treatment process will be diminished.
Another method for phosphorous removal from wastewater involves the formation of struvite, an insoluble magnesium ammonium phosphate salt. Struvite formation in wastewater treatment plants is a relatively common, though unwanted, phenomenon, which can result in clogging wastewater pipes in areas of high flow velocity. Bowers et al. (U.S. Pat. No. 6,994,782) teach a method for the removal of phosphorous through the preferential precipitation and capture of struvite within a desired vessel that contains struvite seed crystals.
In many instances, neutralization is required for proper processing of wastewater. However, neutralization of acidic industrial wastewater can cause insoluble precipitation that under certain circumstance becomes problematic. For example, wastewater that contains a sufficient concentration of sulfur-containing chemicals using calcium containing pH-buffering agent, such as lime, will result in the formation of significant amounts of insoluble calcium sulfate particulates that can be captured in the settleable wastewater solids. The use of lime (Ca(OH)2) for pH neutralization, upon dissolution of lime into calcium cations and hydroxide anions, creates calcium cations that form an insoluble precipitate with sulfate as described in the chemical equations below:Ca(OH)2→Ca2+2OH−Ca2++SO42−→CaSO4 (solid)
The insolubility of calcium sulfate, coupled with the typical long holding time conditions during aeration, results in the formation of CaSO4 particulates that become entrained within the growing matrix of activated sludge present within the secondary treatment process. In a typical secondary treatment process, after a certain amount of aeration time, the activated sludge is pumped into a quiescent zone called a secondary clarifier. In the secondary clarifier, the activated sludge (which is the microorganism biomass that has grown from the consumption of BOD contaminants) is allowed to settle to the bottom and the overflow stream from this clarifier is disinfected and discharged as the final effluent. The long aeration time coupled with the long clarifier settling time results in the accumulation of CaSO4 mineral into the settled activated sludge in a secondary clarifier. A large percentage of the settled activated sludge is returned to the front end of the aeration process, ready to consume more BOD contaminants. This fraction of the activated sludge is called the Return Activated Sludge (RAS). A much smaller percentage of the settled activated sludge is discarded to waste and is thus called Waste Activated Sludge (WAS). The WAS may be mixed with the settled solids that were captured from the primary clarifier and this combined stream is dewatered using any number of standard wastewater dewatering devices, such as a centrifuge, belt press, or screw press. After dewatering, the solids are then sent to a furnace as an inexpensive form of fuel. The high CaSO4 mineral content of the WAS is a significant contributor to the sulfur entering the furnace. Within the furnace, the extremely high temperature can cause the decomposition of CaSO4 to lime and SO2. Thus, the high CaSO4 mineral content of the WAS is a significant contributor to the subsequent emissions of SO2 from the furnace.
Subsequent use of the settled solids as fuel for a furnace will then result in the thermal breakdown of calcium sulfate and release of sulfur dioxide (SO2) as a flu gas into the atmosphere. One such example of an acidic industrial wastewater that contains a sufficient concentration of sulfur-containing chemicals is the wastewater produced from a pulp mill that employs a sulfite pulping process. The flu gas discharge of sulfur dioxide into the atmosphere contributes to negative environmental factors such as acid rain and ozone layer depletion.
There are a number of known methods for SO2 removal or “scrubbing” from flu gases, called flu gas desulphurization (FGD). However, none of the known SO2 removal methods addresses the problem from the root cause, that being to minimize the precipitation and accumulation of sulfur-containing chemicals into the settled wastewater solids.
At a basic level of understanding, the wastewater treatment process involves three primary steps: 1) primary treatment, where settleable solids are removed from the waste stream, 2) secondary treatment, where soluble organic contaminants that did not settle in primary treatment are broken down by microorganisms into beneficial or benign small molecules, such as methane, carbon dioxide, nitrogen, and water, and 3) disinfection, where pathogenic microorganisms present in the final effluent are greatly minimized prior to discharge to a receiving body of water.
Wastewater having a low pH or insufficient alkalinity will not support the effective performance of microorganisms that are needed in the secondary treatment process to consume soluble organic contaminants from the wastewater stream. Typical parameters for measuring the concentration of soluble organic contaminants in a waste stream are biological oxygen demand (BOD) and chemical oxygen demand (COD). Various types of alkaline chemical additives have been employed for increasing the pH and alkalinity into the range where the microorganism population will provide optimum BOD and COD removal performance, typically within a pH range of 6.5 to 8, and more preferentially within the pH range of 7 to 7.5. Neutralization of wastewater is commonly and massively accomplished with sodium hydroxide solutions, a self-defeating process since the excess sodium contained in the treated water makes the treated water less suitable for crop irrigation purposes, and requires additional treatment to reduce sodium content in water.
In view of the foregoing, it is clear that there is a need for new compositions and methods for treating wastewater to remove phosphorous-containing wastewater contaminants.
There is also a need for new compositions and methods to reduce the precipitation and accumulation of sulfur-containing compounds into settled wastewater solids.
There is additionally a need for new compositions and methods to neutralize wastewater that does not employ sodium hydroxide neutralization procedures.